1. Field of the Invention
The present invention relates to power amplifiers for mobile communication devices. More specifically, the invention relates to methods and devices for reducing noise in the power amplifier.
2. Description of the Related Art
Wireless communication is ubiquitous in our modern society because of its convenience and ease of use. FIG. 1 shows a diagram of a typical wireless voice communication device 100, such as a mobile phone handset 100 for cellular telephone use. For the voice interface, device 100 includes a microphone 102 for converting audio signals to electrical signals that are transmitted by transmitter 104. Device 100 also includes a receiver 112 connected to a speaker 114. The transmitter 104 and the receiver 112 normally share an antenna 110, although separate antennas may instead be provided.
The transmitter 104 includes, inter alia, a speech coder 120 that encodes the electrical voice signals before forwarding the encoded signals to a modulator 122 that generates a modulated signal having a carrier frequency at a predefined uplink frequency. Power amplifier 124 amplifies the signal for transmission by the antenna 110. An optional isolator 106 and a receive/transmit duplexer 108 connect the power amplifier/load switch circuit 124 and antenna 110. Using this series of components, the handset 100 may transmit RF signals using the antenna 110.
The receiver 112 obtains a received RF signal from the antenna 110 via the duplexer 108. The received RF signal is also a modulated signal having a carrier frequency at a predefined downlink frequency. The uplink frequency and the downlink frequency differ by a predetermined frequency offset. A low noise amplifier (LNA) 130 amplifies the received RF signal for demodulation. A demodulator 132 demodulates the received RF signal to output a demodulated signal, and a speech decoder 134 decodes the demodulated signal to form an audio signal for reproduction on speaker 114.
The Heterojunction Bipolar Transistor (HBT) is widely used in power amplifiers for cellular communication devices because of its high efficiency and high linearity in the 900 MHz to 1.9 GHz operating range of these devices. HBTs are usually comprised of one or more cells connected in parallel with each cell containing a heterojunction bipolar transistor. As used hereinafter, HBT refers to the collection of cells and the heterojunction bipolar transistor within each cell is referred to as the cell transistor.
HBTs are susceptible to a condition known as thermal runaway or second breakdown wherein one of the HBT cells generates more heat than the other cells in the HBT and becomes hotter than the other cells. The higher temperature of the cell increases current into that cell further increasing its temperature until the cell and the HBT fails.
One common technique used to improve the thermal and electrical stability of HBTs is the addition of a resistor to each of the base nodes of the cell transistors. These resistors are usually referred to as ballast or distributed resistors and reduce the effects of fabrication process variations or design layout effects that lead to non-uniform current distributions among the HBT cells. In general, a larger ballast resistor increases thermal runaway protection but at the cost of a higher bias circuit supply voltage. The higher bias circuit supply voltage decreases the voltage headroom (the difference between the required bias circuit supply voltage and the available voltage from the device's voltage regulator or battery) of the mobile device.
FIG. 2 is a schematic diagram of a typical HBT power amplifier. The diagram of FIG. 2 shows a two-cell HBT 200 for the purposes of illustration only and it is understood that the present invention is not limited to two-cell HBT amplifiers but encompasses HBT amplifiers comprising at least one cell. In FIG. 2, each cell 210, 212 includes a cell transistor, T1, T2, and a ballast resistor, Rb1, Rb2 connected to the base of the cell transistor. Each ballast resistor prevents thermal runaway for its cell transistor but is not necessary for single celled HBTs. The ballast resistors are connected in parallel to each other and in series with a lumped resistor, RL. The lumped resistor may be included, for example, to modify the impedance of the bias and to improve amplifier stability. The lumped resistor may also be eliminated by increasing the values of the ballast resistors. The lumped resistor is connected to a base port 220, which provides a connection to the base biasing network. The collectors of the cell transistors T1, T2 are connected in parallel and terminate at the collector port 230, which provides a connection the collector-biasing network. Blocking capacitors Cin between the RF input port 240 and the base of each of the cell transistors prevent connections between the DC base bias and external DC circuitry connected to port 240. Similarly, blocking capacitor Cout between the HBT collector and the RF output port 250 prevents connections between the DC collector bias and external DC circuitry connected to output port 250.
In addition to amplifying the RF signal applied to input port 240, the HBT amplifies noise signals transmitted to the base of the HBT. Noise signals may be present at the input of the amplifier or may arise from the base biasing network connected to base port 220. Ballast resistors also generate noise that is amplified with the RF signal input to the HBT. Regardless of the noise source, the noise at the base of the HBT may mix up or up-convert, such that the amplified signal spills into nearby channels as noise such as, for example, the receive band of the mobile device. Therefore, there remains a need for the reduction of noise from a signal applied to the base of an HBT amplifier.